1. The Field of the Invention
The present invention relates to the formation of high aspect ratio submicron VLSI contacts. More specifically, the present invention is directed to depositing germanium from germane gas using an LPCVD process into a contact opening in order to remove native silicon dioxide from the contact opening. The layer of germanium in the bottom of the contact opening is consumed during annealing to form a low resistance contact.
2. The Relevant Technology
The movement toward progressive miniaturization of semiconductor devices has resulted in increasingly compact and efficient semiconductor structures. This movement has been accompanied by an increase in the complexity and number of such structures aggregated on a single semiconductor integrated chip. As feature sizes are reduced, new problems arise which must be solved in order to economically and reliably produce the semiconductor devices. The submicron features which must be reduced include, for instance, the width and spacing of metal conducting lines as well as the size of various geometric features of active semiconductor devices.
As an example, the requirement of submicron features in semiconductor manufacturing has necessitated the development of improved means of making contact with the various structures. The smaller and more complex devices are achieved, in part, by reducing device sizes and spacing and by reducing the junction depth of regions formed in the semiconductor substrate. Among the feature sizes which are reduced in size are the contact openings through which electrical contact is made to active regions in the semiconductor devices. As both the contact size and junction depth are reduced, new device metallization processes are required to overcome the problems which have been encountered.
Historically, device interconnections have been made with aluminum or aluminum alloy metallization. Aluminum, however, presents problems with junction spiking. Junction spiking results in the dissolution of silicon into the aluminum metallization and aluminum into the silicon. Typically, when aluminum contacts with the silicon substrate directly, the aluminum eutectically alloys with the silicon substrate at temperatures lower than 450.degree. C. When such a reaction occurs, silicon is dissolved into the aluminum electrode, and there is a tendency for silicon thus dissolved into the electrode to be precipitated at a boundary between the electrode and the substrate as an epitaxial phase. This increases the resistivity across the contact. Furthermore, aluminum in the electrode is diffused into the silicon substrate from the electrode and forms an alloy spike structure in the substance.
The resulting alloy spike structure is a sharp, pointed region enriched in aluminum. The alloy spikes can extend into the interior of the substrate from the boundary between the electrode and the substrate to cause unwanted short circuit conduction at the junction of the semiconductor in the substrate, particularly when the junction is formed in an extremely shallow region of the substrate. When such an unwanted conduction occurs, the semiconductor device no longer operates properly. This problem is exacerbated with smaller device sizes, because the more shallow junctions are easily shorted, and because the silicon available to alloy with the aluminum metallization is only accessed through the small contact area, increasing the resultant depth of the spike.
Contact openings have also been metallized with chemical vapor deposited tungsten. This process has also proven problematic. The tungsten is typically deposited in an atmosphere of fluorine, which attacks the silicon, creating "wormholes" into the active region. Wormholes can extend completely through the active region, thereby shorting it out and causing the device to fail. Tungsten also presents a problem in that it does not adhere well directly to silicon.
3. Prior State of the Art
In order to eliminate the problems associated with the reaction between the silicon substrate and the metallization material, prior art solutions have typically used a diffusion barrier structure in which the reaction between the silicon substrate and the electrode is blocked by a barrier layer provided between the electrode and the substrate. Such a barrier layer prevents the diffusion of silicon and aluminum. It also provides a surface to which the tungsten will adhere and which will prevent tungsten and fluorine from diffusing into the active region.
Prior art FIGS. 1 through 4 of the accompanying illustrations depict one conventional method known in the art of forming contacts having a diffusion barrier. In FIG. 1, a contact opening 18 is etched through an insulative layer 16 overlying an active region 14 on a silicon substrate 12. Insulating layer 16 typically comprises a passivation layer of intentionally formed silicon dioxide in the form of borophosphosilicate glass (BPSG). Contact opening 18 provides access to active region 14 by which an electrical contact is made. Layer 20 is a thin native oxide layer which forms on the active region from exposure to ambient. As shown in FIG. 2, a titanium metal layer 22 is then sputtered over contact opening 18 so that the exposed surface of active region 14 is coated.
A high temperature anneal step is then conducted in an atmosphere of predominantly nitrogen gas (N.sub.2). Native oxide layer 20 is dissolved and titanium metal layer 22 is allowed to react with active region 14 and change titanium metal layer 22 into a dual layer. As shown in FIG. 3, a layer of titanium silicide (TiSi.sub.x) 26 is formed by the anneal step, and provides a conductive interface at the surface of active region 14. A layer of titanium nitride (TiN.sub.x) 24 is also formed, and acts as a diffusion barrier to the interdiffusion of tungsten and silicon or aluminum and silicon, as mentioned above. Under such conditions, the lower portion of titanium metal layer 22 overlying active region 14, after dissolving layer 20, reacts with a portion of the silicon in active region 14 to form titanium silicide region 26. Concurrently, the upper portion of titanium metal layer 22 reacts with the nitrogen gas of the atmosphere to form titanium nitride layer 24.
The next step, shown in FIG. 4, is metallization. This is typically achieved by chemical vapor deposition (CVD) of tungsten, or by the deposition of aluminum using any of the various known methods. These include aluminum reflow sputtering, and chemical vapor deposition. In the case of tungsten, the titanium nitride helps improve the adhesion between the walls of the opening and the tungsten metal. In the case of both tungsten and aluminum, the titanium nitride acts as a barrier against the diffusion of the metallization layer into the diffusion region and vice-versa.
Spiking and wormholes can still occur, even with the use of a deposition barrier, particularly when the diffusion barrier is too thin. This frequently occurs at the comers of the contact opening, where it is difficult to form a thick layer, particularly if the aspect ratio of the contact is high. Contact opening 18 of FIG. 3 is filled by a layer of aluminum 32 in FIG. 4 which depicts the effects of spiking, with a spike 34 extending through active region 14, the effect of which is to short active region 14 out.
The compound titanium nitride (TiN) is well suited to forming a diffusion barrier, as it is extremely hard, chemically inert, an excellent conductor, and has a high melting point. It also makes excellent contact with other conductive layers. Titanium nitride is typically formed by the reaction of sputtered titanium during annealing in nitrogen, or can be deposited directly on the substrate by reactive sputtering, evaporation, chemical vapor deposition and the like before the deposition of the metallization.
As device dimensions continue to shrink and the contact openings become deeper and narrower, contact walls become vertical and most of the metal deposition techniques fail to provide the necessary step coverage to create adequate contact with the active area. Such narrow, high aspect ratio contact openings can result in a partial or total failure to make significant contact with the active region. Accordingly, it becomes increasingly difficult to produce the desired thickness of titanium at the bottom of the contact opening.
FIG. 5 shows the dimensions used to calculate the aspect ratio, which is the ratio of the height H to the width W. In order to introduce a sufficiently thick layer of titanium 22 using conventional sputtering techniques and thereby create an effective diffusion barrier 50, the aspect ratio of contact opening 18 is required to be kept relatively low, generally under 2:1.
The aspect ratios of contacts have been increased in the past by depositing the titanium layer using a collimator to directly sputter deposit plasma emanating from a target into the bottom of the contact openings on a silicon wafer. The use of a collimator to direct titanium layer metal 22 in FIG. 2 to the bottom of contact opening 18 prevents unwanted structures from forming on the walls of contact opening 18 and thereby plugging contact opening 18.
A collimator having a honeycomb structure has an aspect ratio corresponding to the diameter of the openings in the honeycomb structure divided by the thickness of the honeycomb structure. In order to deposit the thick layers of titanium needed for this conventional method, the honeycomb structure used in collimator sputtering has been required to have a high aspect ratio, typically around 2.5:1. This slows down the manufacturing process and reduces throughput. Higher aspect ratios also require a high surface area of the collimator. A consequence of a high surface area is a concomitant increase in particle contamination, and a reduced deposition ratio on the wafer.
Other undesirable effects result from the conventional contact forming method. For instance, a high temperature of 800.degree. C. or greater is required during the anneal step to properly form titanium silicide region 26 as shown in FIG. 3. In practice, high temperatures tend to cause loss to the titanium silicide layer and can cause the BPSG to crack and to reflow.
Another function of depositing a titanium layer in a contact opening is to remove native silicon dioxide (SiO.sub.2) which forms whenever the wafer is exposed to air. Typical native silicon dioxide layers have a thickness of about 20 Angstroms. Such a layer is shown at 20 in FIG. 1. Native silicon dioxide layer 20 is highly insulative and can cause a high contact resistance so as to result in failure of the device. Titanium layer 22 of FIG. 2 serves to carry away oxygen, breaking down silicon dioxide layer 20. In the process, a portion of titanium layer 22 is consumed. As a result, even more titanium must be deposited in order to form an effective diffusion barrier.
Prior art methods employed plasma cleaning to remove the silicon dioxide from the bottom of the contact openings prior to depositing titanium. These processes have proven unsatisfactory, as they are quite expensive, decrease throughput, and may require substantially higher RTP annealing temperatures. Furthermore, since native silicon dioxide grows in air, these methods do not prevent the reformation of native silicon dioxide in the contact openings once the methods are concluded.
For these reasons, there is a need in the art for an improved method of creating diffusion barriers in contacts that minimize the amount of material needed for effective diffusion barriers. This will in turn allow greater miniaturization of devices. Such a method would be more desirable if it also had increased throughput, lowered costs, and increased yields.